Set the ink limits the same value for the 2 channels you want to use, and set the rest of the channels to 0.
Set the N_OF_GRAY_PARTS=1
Set
GRAY_INK_1=K
GRAY_VAL_1=100
Make the gray_highlight/shadow=0 as Greg said (to get a straight line from 0-100), but instead of making both cross over points 100 make the at the end of the descriptor file paste in COPY_CURVE_(the ink you're using)=K
If you try to set both inks to use the same gray val then one channel gets canceled out. This is the case if you set both inks to GRAY_INK_1= and GRAY_VAL_1=100 as well as setting the second ink to GRAY_INK_2= and GRAY_VAL_2=100. In the text based graph it might look like the C is on top of the K curve, but if you open the quad file CurveView you'll find that only one of the inks is being used (there is only 1 ink with an ink limit that is not "0" )
I pasted an example ink descriptor file below which should work for what you need it to do. You will need to make a few changes depending on the number of inks your printer uses, and the ink limit that you want to print with. I stripped out all the stuff that is not needed and there shouldn't be a problem with it if you're working on a Mac.
Hope that helps,
Richard Boutwell
PRINTER=Quad1430-k6
CURVE_NAME=Shade2on1
GRAPH_CURVE=YES
N_OF_INKS=6
DEFAULT_INK_LIMIT=0
LIMIT_K=75
LIMIT_C=75
LIMIT_M=0
LIMIT_Y=0
LIMIT_LC=0
LIMIT_LM=0
# GRAY VALUE CROSS-OVER POINTS
N_OF_GRAY_PARTS=1
GRAY_INK_1=K
GRAY_VAL_1=100
# GRAY CURVE INFORMATION
GRAY_HIGHLIGHT=0
GRAY_SHADOW=0
GRAY_OVERLAP=0
GRAY_GAMMA=1
GRAY_CURVE=
# Ink Curves
COPY_CURVE_C=K